This invention relates to an eye pattern or signal point constellations which is generated upon transmission and also to a modulation and demodulation method which employs the eye pattern.
Modems (modulator-demodulators) are generally used widely for transmission of data in analog circuits which make use of a voice band.
Some modems make use, in addition to a main channel for transmitting main data of a terminal or a like element, of a secondary channel provided by frequency division for transmitting signal quality information, a reception level and some other information to supervise the state of the network.
FIG. 15 shows a general construction of an on-line system. Referring to FIG. 15, in the on-line system shown, a plurality of modems 203 are connected to a host computer 201 by way of a communication control apparatus (CCP) 202, and each of the modems 203 is connected by way of an analog circuit 204 to another modem 203' installed at another location. A terminal 205 is connected to each of the models 203'.
The on-line system further includes a network supervisory apparatus 206, for which a secondary channel is used.
By the way, a state signal of a modem can be transmitted, from each of the host side modems 203 shown in FIG. 15, as it is to the network supervisory apparatus 206, but from each of the terminal side modems 203', a state signal thereof is transmitted to the associated host side modem 203 so that it is transmitted by way of the modem 203 to the network supervisory apparatus 206.
Since a state signal of a modem must necessarily be transmitted without having any influence on main data, each of the modems 203 and 203' divides, for example, a voice band of 0.3 kHz to 3.4 kHz by frequency division to provide a secondary channel for secondary data in addition to a main channel for main data as seen in FIG. 16.
It is to be noted that phase shift keying (PSK), orthogonal amplitude modulation (OAM) or some other modulation is used for a main signal while frequency shift keying (FSK) is used for a secondary signal.
Meanwhile, a modem generates, upon transmission, signal points having a predetermined eye pattern (data point arrangement pattern on a phase plane) to modulate and transmit main data and secondary data and demodulates, upon reception, a reception signal to reproduce such main data and secondary data. When, for example, the transmission rate is 96 bps and the modulation rate is 48 Baud, the eye pattern of the secondary channel has an arrangement of points which is symmetrical with respect to the origin of the signal point arrangement plane as shown in FIG. 17.
By the way, it is required for modems in recent years to establish multiple point connection in addition to a rise of the communication rate to reduce the cost of the circuit. To this end, it is an effective technique to divide a frequency band of a main channel into a plurality of sub-bands to transmit a plurality of data by way of the same circuit. However where the technique is employed, since the roll-off ratio of the main channel is reduced very low, it is difficult to extract a timing component from the main channel. Therefore, it is necessary to extract a timing component from the secondary channel which has a high roll-off ratio.
However, when a timing component is extracted from the secondary channel in this manner, if the secondary channel has a jitter of, for example, 1% as seen in FIG. 18(a), then the jitter amount is 200 .mu.sec, which does not matter at all when viewed from the secondary channel. However, when the jitter amount is viewed from the main channel wherein, for example, the transmission rate is 4,800 bps and the modulation rate is 2,400 Baud, since this corresponds to a jitter of 50% as seen from FIG. 18(b), there is a subject in that the jitter amount is very great to the secondary channel.
In particular, the jitter amount of 1% as viewed from the main channel is 4 .mu.sec, and from this, if it is set that the required jitter amount is 4 .mu.sec, then this corresponds to the jitter amount of 0.02% as viewed from the secondary channel. Consequently, the allowable jitter amount is very little. Thus, if the jitter amount of 0.02% must be realized, then also the arrangement of an eye pattern must be an ideal arrangement.
Further, where the signal point arrangement is such as shown in FIG. 17, when there is a frequency offset of 12 Hz, the rotational angle is 90.degree. (360.degree..times.12/48), and consequently, the phase is stabilized at points displaced by 90.degree.. As a result, it is determined in error that there is no offset, and consequently, it is difficult to follow up the offset.